Process for forming of stretched sheet material



y 1962 R. J. CLAPP ETAL 25,202

PROCESS FOR FORMING OF STRETCHED SHEET MATERIAL Original Filed Oct. 28,1955 5'7 :fIIIIIII/IIIII/III/lVII/IIII/IIdZf7/IIIIII/IIIIIIIIIIlIIIIlII/fi 11 IIII/IIIIIIIIIII 1/1 ITTOP/Vt'VSUnited States Patent once Re. 25,202 Reissued July 17, 1962 Original No.2,897,546, dated Aug. 4, 1959, Ser. No.

543,499, Oct. 28, 1955. Application for reissue July 22, 1960, Ser. No.73,469

Claims. (Cl. 18-56) Matter enclosed in heavy brackets I: appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the addition made by reissue.

This invention relates to the fabrication of plastics in sheet form andhas particular reference to a process for forming stretchedthermoplastic sheet material.

Recent advances in the production o-faircraft glazing materials includethe development of stretched monolithic sheet materials made oftransparent thermoplastics. These stretched sheets have improvedproperties of toughness and resistance to crazing and fracturing. Bysubjecting thermoplastic sheets to linear stretching under certainconditions, the shatter-resistance properties of the sheets areimproved, due apparently to the fact that a laminar structure is set upin the sheet. The stretching operation results in a material havingshatter-resistant properties similar to that of laminated sheeting suchas conventional safety glass Without suffering the weight penalty of thelaminated products. parent plastics present a new material which isespecially suitable for use in aircraft glazing applications where lowweight plays an extremely important part.

Suitable techniques have heretofore been developed for stretchingthermoplastic sheeting, either biaxially, i.e., in two, mutuallyperpendicular directions, or multiaxially, i.e., in a plurality ofdirections extending radi-.

ally outwardly from the center of the sheet, and in this specificationthe term stretched or similar terms is intended to comprehend either ofsuch types of stretching. However, many problems have arisen'inconnection with the production of stretched materials formed to thecurved contours usually required in aircraft glazing applications. Oneof the principal objects of this invention is, therefore, to provide amethod for forming stretched thermoplastic sheeting into desired curvedconfigurations.

Another object of this invention is to provide a process for formingfiat stretched thermoplastic sheeting into both simple and compoundcurve configurations without significantlydegrading the toughness of thesheeting in the asstretched condition.

Still another object of this invention is to provide a process forcontact forming flat stretched thermoplastic sheet material into bothsimple and compound configurations without mark-off from the die ontothe sheet such as to impair the optical properties of the sheet.

Another object of the invention is to provide a forming process forstretched thermoplastic sheet material, which utilizes conventionaltooling familiar to those skilled in the art of plastics fabrication.

' Other objects and advantages of this invention it is believed'will bereadily apparent from the following detailed description of preferredembodiments thereof when read in connection with the accompanyingdrawings.

In the drawings:

FIGURE 1 is an exploded perspective view illustrating one form ofapparatus utilized in carrying out the process of this invention.

of FIGURE 1.

Thus, stretched trans- FIGURE 3 is a sectional elevation takensubstantially on the line 33 of FIGURE 2 and illustrating an initialstage of the process.

FIGURE 4 is a view similar to FIGURE trating a final stage of theprocess.

Briefly, this invention includes the discovery that flat stretchedthermoplastic sheeting can be formed to simple or compound curvedconfigurations by mechanically forcing the stretched sheet to conform tothe desired curved configuration and holding the sheet in this positionwhile heating the sheet to a temperature high enough to relieve thestrains imposed by the mechanical forming operation, and preferablyclosely approaching, but not substantially above, a critical temperatureat which substantial shrinkage of the thermoplastic would take place. Ithas been found that during the stretching operation, residual stressesare set up in the sheet and that if the sheet is warmed to the formingor thermoplastic temperature of the particular thermoplastic materialused, the sheet will completely shrink-back, i.e., return to itsoriginal, as-cast dimensions. It has further been found that therelationship between temperature and amount of shrink-back is: such thatthere is a critical temperature, for any given thermoplastic material,at which appreciable shrink-back begins and that, while this temperatureis below the normal forming or thermoplastic temperature for thatparticular material, it isof considerable magnitude, relative to roomtemperatures. In other words, it has been found that an appreciabledegree of heat may be applied to the stretched material before an amountof shrinkback takes place which could be considered to be significantwith respect to causing loss of toughness in the stretched sheet. It hasbeen found, also, that superior results are obtained if the temperatureof the part be raised,

' while it is in the restrained, contoured configuration, to atemperature such that a small amount of shrink-back takes place. It isbelieved that this induced shrink-back of the material plays a part, atleast, in locking the thermoplastic into the desired curvedconfiguration. This temperature, as indicated above, varies dependingupon the particular thermoplastic material being utilized, but it hasbeen found that best results are obtained if the temperature ismaintained in a range such as to produce a measurable amount ofshrink-back in a flat stretched specimen but no more about 3 percentshrinkage.

By the term percent shrinkage and similar terms used ration of thepartto be formed. This surface is covered a with a sheet of rubberized suede12. The die is provided ing a rubberized suede surface 21 which mateswith a continuous portion of the male die. A plurality of C-clamps 25serve to hold the male and female dies together.

In carrying out the process, the male and female dies are preferablypreheated to bring the entire mass thereof up to the desired formingtemperature, several hours generally being required to accomplish this.Preferably the flat stretched sheet 30 is also preheated. In some casesit is desirable to preheat the sheet to the forming tern 3, butillusperature, but for best results, the sheet should not be maintainedin this temperature while in the flat condition for a length of timewhich would be sufficient to bring about substantial shrinkage.Generally it is sufiicient to preheat the flat sheet only for asufficient length of time to warm the surfaces thereof purely toavoidany surface crazing' of the material during the mechanical formingoperation. In this case, the body of the plastic sheet remainssubstantially at room temperature and while at this temperature thesheet is placed between the male and female dies and the C-clamps arepartially turned down so as to partially form the sheet, as shown inFIGURE 3. The desired forming temperature is then applied to theassembly for a period of time to at least partially relieve the stressesset up by this degree of deformation of the sheet. Usually about /2 houris sufiicient. Thereupon, if, as is the case with the dies shown, theconfiguration is not of relatively small radii, the clamps are turnedall the way down to force the sheet 30 flush with the die surfaces, asshown in FIGURE 4. The assembly is again subjected to the desiredforming temperature and held at this temperature for a sufficient lengthof time to assure that the entire mass of the sheet is heated uniformlyand to assure that all residual stresses set up by the forming arerelieved. This time factor varies, depending upon the actual formingtemperature, lower temperatures requiring correspondingly longer timeperiods, and vice versa. The heating period depends also upon the sizeof the part, the mass of the die, etc., but generally from about 4-20hours is sufiicient.

After the heating period, the die and the part are cooled, bycirculation of cooling water, to room temperature, and the clamps arethen removed. Usually about 4-8 hours of such cooling is sufiicient.

As indicated above, the actual forming temperature varies, dependingprimarily upon the type of thermoplastic material being operated on. Inorder to obtain relatively simple contours, the temperature may be quitelow-no more than is required to relieve the externally induced stressesset up by the mechanical forming of the sheet. However, the optimumtemperature appears to be just at or slightly below the criticaltemperature at which appreciable shrink-back begins. For example, in thecase of stretched cast polymethyl methacrylate such as stretchedPlexiglas II (Rohm & Haas) or its equivalent, specified as stretchedMIL-P-5425A, the optimum forming temperature is about 185 -F. Theoptimum forming temperature for stretched, modified (partiallycross-linked) polymethyl methacrylate such as stretched Plexiglas 55(Rohm & Haas or its equivalent, specified as stretched MIL-P-8184, is215 F. The preferred forming temperature for stretched polymethylalpha-chloracrylate such as stretched Gafite (General Aniline & FilmCorp.) or its equivalent, specified as MIL-P8427 (USAF tentative), is245 F. By way of comparison, normal forming temperature for as-castPlexiglas II is above 275 F.; Plexiglas" 55, above 300 F.; and Gafite,above 350" F.

In practicing the invention, it is preferred to carry out the entireprocess, including the clamping operation, in a large oven, although, ifdesired, a relatively small oven or other heating means may be employed,and the die assembly removed for each clamping operation. For extremelysimple contours of large radii, only a single clamping operation need beutilized, but for more sharp contours, it is preferred to bring the partto the ultimate contour in a series of gradual steps so as to avoidoverstrain which might result in crazing, fracture or other physicaldamage. It will be understood by those skilled in the art that othertypes of apparatus may be used in carrying out the process of thisinvention such as, for example, a hydraulic ram for the female die, useof both female and male full form dies, hydraulic bag techniques witheither a male or female form die, etc. The only re- 4 qui-rement is thatsome means he provided for mechanically'bending or forcing the stretchedsheet into a configuration and holding it in such configuration whilethe forming heat is applied thereto, and during the cooling period.

An' important feature of the invention, aside from the obviouslyimportant result of forming stretched sheets to desired contours withoutsubstantial loss in toughness, resides in the fact that acrylic andother thermoplastic materials may be formed to accurate contours bycontact forming techniques, yet without obtaining any mark-off from thedies which would impair the optical properties of the'finished part.Thus, with the process of this invention, all types of transparentaircraft enclosures, such as canopies, windshelds, windows and the likecan be fabricated from stretched thermoplastics.

The following specific examples of the process are illustrative of theinvention, but it is to be understood that the invention is not to belimited to the details set forth there- EXAMPLE 1 Part No. 84, a sheetof Plexiglas 55, 49" x 49" x 0.350" in size, which had been stretched61.6% in one direction and 60.5% in the other direction, was formed intoa compound-curved aircraft window as follows:

A solid male form die and a female ring die were used,

and this apparatus was preheated for 14 hours at 170 F.

The mold and stretched sheet were then placed in an oven at 215 F. forabout 1% hours. During this preheating period the sheet rested on themale die and the female die rested upon the sheet, in readiness forclamping; After the 1 /2 hour period, the clamps were applied and thefemale die brought down a short distance to impart a small degree ofchange of configuration in the sheet. Thereafter, the clamps weretightened every /2 hour with the final clamping to bring the sheet flushwith the die surfaces taking place 4 hours after clamping was begun. Allthis time the 215 F. temperature was maintained, and the oven and theassembly were maintained at this temperature, with some fluctuationbelow this temperature, for an additional 12 hours, at which time theheating was discontinued.

Upon discontinuing of the heating, the assembly was covered with aflannel blanket to avoid surface chill, and the oven doors opened. Afterseveral hours, the mold temperature had cooled to 110 F. and the clampswere removed. The finished part Was found to be formed generally to thedesired contour.

The average shrinkage of ten flat specimens taken from the originalstretched sheet and exposed to the same temperature cycle was 0.167%.The average dW/dA value (a measure of toughness comprising work per unitarea required to propagate a crack, expressed as inch-pounds per squareinch) of several specimens of the as-stretched material was 22.6,whereas the dW/dA of the flat specimens subjected to the formingtemperature conditions was 24.2. Thus, there was no loss of toughnessdue to the forming conditions, and, in fact, there was an apparentincrease.

EXAMPLE 2 Part No. 90, a sheet of Plexiglas 55, 40 x 48" x i 0.350",which had been stretched 67.3% in one direction and after dW/dA valueswere 30.0 and 23.2. There was some obvious loss in toughness, but thisis not significant when it is considered that dW/dA values for theas-cast material is in the neighborhood of 3-5.

EXAMPLE 3 Part No. 88, a sheet of P-lexiglas" 55, 50" x 50" x 0.350",which had been stretched 58.7% in one direction and 65.7% in the otherdirection was formed to satisfactory contour under substantially thesame conditions as set forth in Example 2.

Average shrinkage of the flat specimens was 0.292%. The before and afterdW/dA values were 29.8 and 29.5.

EXAMPLE 4 EXAMPLE 5 Part No. 112, a sheet of Plexiglas 55, 50" x 50" x0.350", which had been stretched 67.3% in one direction and 62.3% in theother, was formed with a compoundcurved aircraft window as follows:

A thin-shell phenolic die, such as the die 10, and a female ring, suchas the ring 20, were preheated for 6 hours at 170 F. and at 215" F. forone hour. The sheet was then placed in the mold, without clamping, andsubjected to the 215 F. oven heat for one hour. The clamps were appliedand turned down once an hour for two hours. The assembly remained in theoven at 215 F. for an additional 12 hours whereupon it was blanketed,removed from the oven and water cooled for about 8 hours. After thiscooling, the clamps were removed and the part was found to be formed tocontour.

The average shrinkage of flat specimens was 1.160%. The before and afterdW/dA values were 29.6 and 29.8.

While the process of this invention has been described as particularlyapplied to the forming of acrylics, it will be understood that it isapplicable to all thermoplastics which can be stretched, such as, forexample, polystyrene.

Having fully described our invention, it is to be understood that we donot wish to be limited to the details set forth, but our invention is ofthe full scope of the appended claims.

We claim:

1. A process for forming flat stretched thermoplastic sheet material oftough laminar structure into curved contours without substantialdecrease in toughness and resistance to crazing and fracturing of suchmaterial, comprising the steps of mechanically forming the sheet atgatemperature below the thermoplastic temperature of said material to acurved contour while imparting stresses therein, holding the sheet tosaid contour while heating the same to a temperature suflicientlyhighand for a sufiicient length of time to impart a small amount ofshrink-back to the sheet, said shrink-back corresponding to a measurableamount of shrinkage in a flat specimen of said stretched sheet subjectedto the same temperature conditions as said contoured sheet, but belowthe temperature at which more than about 3 percent shrinkage of saidflat specimen occurs, and cooling said sheet while continuing to holdthe sheet to said contour.

2. A process for forming flat stretched acrylic sheet material of toughlaminar structure into curved contour-s without substantial decrease intoughness and resistance to crazing and fracturing of such material,comprising the steps of mechanically forming the sheetat a temperaturebelow the thermoplastic temperature of such sheet material to a curvedcontour while imparting stresses therein, holding the sheet to saidcontour while heating the same to a temperature sufliciently high andfor a sufiicient length of time to impart a small amount of shrink-backt0 the sheet, said shrink-back corresponding to a measurable amount ofshrinkage in a flat specimen of said stretched sheet subjected to thesame temperature conditions as said contoured sheet, but below thetemperature at which more than about 3 percent shrinkage of said flatspecimen occurs, and cooling said sheet while continuing to hold thesheet to said contour.

3. A process for forming flat stretched polymethyl methaorylate sheetmaterial of tough laminar structure into curved contours withoutsubstantial decrease in the toughness and resistance to crazing andfracturing of such material, comprising the steps of mechanicallyforming the sheet at a temperature below the thermoplastic temperatureof such sheet material to a curved contour While imparting stressestherein, holding the sheet to said contour while heating the same to atemperature of about 185 F. for a suflicient length of time to set thesheet in the curved contour, and to impart a small amount of shrink-backto the sheet, said shrink-back corresponding to a measurable amount ofshrinkage, but not more than about 3%, in a flat specimen of saidstretched sheet subjected to the same temperature conditions as saidcontoured sheet and cooling said sheet while continuing to hold thesheet to said contour.

4.-A process for forming flat stretched, cross-linked polymethylmethacrylate sheet material of tough laminar structure into curvedcontours without substantial decrease in the toughness and resistance tocrazing and fracturing of such material, comprising the steps ofdeforming the sheet material at a temperature below the thermoplastictemperature of such sheet material and to a curved contour whileimparting stresses therein, holding the sheet to said contour whileheating the same to a temperature of about 215 F. for a suflicientlength of time to set the sheet in the curved contour, and to impart asmall am unt of shrink-back to the sheet, said shrink-back correspondingto a measurable amount of shrinkage, but not more than about 3%, in aflat specimen of said stretched sheet subjected to the same temperatureconditions as said contoured sheet and cooling said sheet whilecontinuing to hold the sheet to said contour.

5. A process for forming flat stretched polymethyl alpha-chloracryl'atesheet material of tough laminar struc ture into curved contours withoutsubstantial decrease in the toughness and resistance to crazing andfracturing of such material, comprising the steps of mechanicallyforming the sheet at a temperature below the thermoplastic temperatureof such sheet material to a curved contour while imparting stressestherein, holding the sheet to said contour while heating the same to atemperature of about 245 F. for a suflicient length of time to set thesheet in the curved contour, and to impart a small amount of shrink-backto the sheet, said shrinkback corresponding to a measurable amount ofshrinkage, but not more than about 3%, in a fiat specimen of.

said stretched sheet subjected to the same temperature conditions assaid contoured sheet and cooling said sheet while continuing to hold thesheet to said contour.

6. A process for forming flat stretched thermoplastic sheet material oftough laminar structure into curved contours without substantialdecrease in the toughness and resistance to crazing and fracturing ofsuch material, comprising the steps of mechanically forming the sheet toa curved contour while at least the main body of the sheet is maintainedat approximately normal room temperature, and holding the sheet to saidcontour while heating the same to a temperature sufliciently high andfor a sufiicient length of time to impart a small amount of shrink-backto the sheet, said shrink-back corresponding to a measurable amount ofshrinkage in a flat specimen of said stretched sheet subjected to thesame temperature conditions as said contoured sheet, but below thetemperature at which more than about 3 percent shrinkage of saidflatspecirnen occurs.

7. A process for forming fiat stretched acrylic sheet material of toughlaminar structure into curved contours without substantial decrease'inthe toughness and resistance to crazing and fracturing of such material,comprising the steps of mechanically forming the sheet to a curvedcontour while at least the main body of the sheet is maintained atapproximately normal room temperature, holding the sheet to saidcontour. while heating the same to a temperature sufiiciently high andfor a sufficient length of time to impart a small amount of shrink-backto the sheet, said shrink-back corres onding to a measurable amount ofshrinkage in a flat specimen of said stretched sheet subjected to thesame temperature conditions as said contoured sheet, but below thetemperature high and forasufiieien't length of time to produce saidshrinkage, and cooling said sheet while continuing to hold the sheet tosaid contour.

9. The process of claim 8 wherein the sheet material is olymethylmethacry'late.

10. The process of claim- 8 wherein the mechanical forming step iscarried out while at least the main body of the sheet is maintained atapproximately normal room temperature.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS 2,330,837 Mullen Oct. 5, 1943 2,370,827Weichbrod-t et al. Mar. 6, 1945 2,444,420 Borkland, July 6, 19482,493,439 Braund Jan. 3, 1950 2,659,105 Halbig Nov. 17, 1953 2,688,772.Sandorfi Sept. 14, 1954 2,759,217 Peterson Aug. 21, 1956 OTHERREFERENCES Styrene, Its Polymers, Copolymers and Derivatives(Boundy-Boyer), published by Reinhold Publishing Corp, New York,1952,'pages 600-606, 11584161. (Copy in Scientific Library.)

